The photoreceptors of Limulus ventral eye, like all other photoreceptors, can undergo large changes in their sensitivity to light. The purpose of this project is to understand the mechanism of these sensitivity changes. Recent work on the early receptor potential (ERP) indicates that Limulus visual pigment is similar to that in the squid. To determine if there is a photochemical component of adaptation, sensitivity can be measured before and after alteration of pigment states. Preliminary results indicate that there is probably a small photochemical component of adaptation, but that the primary mechanism of adaptation is neural. The mechanism of neural adaptation is also being studied. Changes in the level of intracellular calcium have been implicated as important in the regulation of sensitivity. We hope to determine how calcium is released into the cell, and where it might act. By studying the visual pigment of these cells, it may be possible to determine if there is a special class or subclass of visual pigment responsible for the demonstrated intracellular release of calcium. Work in the laboratory also deals with identification of the ionic mechanisms underlying the non-linear current-voltage curves of the dark membrane. BIBLIOGRAPHIC REFERENCES: Lisman, J.E., and J.E. Brown. 1975. Light-Induced Chnges of Sensitivity in Limulus Ventral Photoreceptors. J. Gen. Physiol. 66: 473. Lisman, J.E., and J.E. Brown. 1975. Effects of Intracellular Injection of Calcium Buffers on Light Adaptation in Limulus Ventral Photoreceptors. J. Gen. Physiol. 66:489.